"While we believe the majority of people have received the update, we recognize that not all have," Microsoft's Brandon LeBlanc wrote in a blog post. "Having our customers running their devices with the latest updates is super important to us. And we're committed to helping ensure their safety."

To be clear, users of Windows 8, Windows 7, and Windows Vista will continue to get security patches. The update requirement only applies to Windows 8.1 users, who need to download the hefty update that Microsoft delivered in April. You can see whether you're running Windows 8 or Windows 8.1 by going to PC Settings > PC and devices > PC info and looking under the "Windows" section.

Most Windows 8.1 users should have received this update automatically, but some users may have disabled automatic updates and could still be running an older version of Windows 8.1. To check which version of Windows 8.1 you have, simply go to the Start screen, and look for the search button in the top-right corner. As long as you see that button, you're running the latest version.

Users who don't have the latest update should go to Settings > Change PC Settings > Update and recovery > Windows Update. Select "Check now," and if updates are found, select "View details." Install the update labeled KB 2919355. If you don't see that option, look for KB 2919442 and install it first.

Microsoft warns that Windows 8.1 users will miss out on security patches, bug fixes and new features if they don't install this update. But again, this requirement doesn't apply to users of Windows 8 or earlier. Additional details and troubleshooting tips can be found on Microsoft's Website.